We present spectroscopic evidence for the creation of entangled macroscopic quantum states in two current-biased Josephson-junction qubits coupled by a capacitor. The individual junction bias currents are used to control interaction between tuning energy level spacings junctions and out resonance with each other. Microwave spectroscopy 4 6 gigahertzrange at 20 millikelvin reveals levels that agree well theoretical results states. single spatially separate, extend over 0.7-millimeter distance qubits.

We study the quantum mechanical behavior of a macroscopic, three-body, superconducting circuit. Microwave spectroscopy on our system, resonator coupling two large Josephson junctions, produced complex energy spectra well explained by theory over frequency range. By tuning each junction separately into resonance with resonator, we first observe strong between and resonator. Bringing both junctions together find spectroscopic evidence for entanglement all 3 degrees freedom suggest new method...

The zero-voltage state of a Josephson junction biased with constant current consists set metastable quantum energy levels. We probe the spacings these levels by using microwaves to induce transitions and thereby enhance escape rate voltage state. widths resonances give measurement spectroscopic coherence time two states involved in transitions. observe decoherence shorter than that expected from dissipation alone resonantly isolated...

We show that two capacitively coupled Josephson junctions, in the quantum limit, form a simple qubit system with effective coupling controlled by junction bias currents. compute numerically energy levels and wave functions for system, how these may be tuned to make optimal qubits. The dependence of on parameters can measured spectroscopically, providing an important experimental test presence entangled multiqubit states Josephson-junction based circuits.

We present a design for effectively isolating low-dissipation superconducting tunnel junctions without causing excessive heating. In order to obtain long decoherence time in macroscopic quantum coherence experiments, it will be essential make high impedance connections the junction. our design, are made by thin-film resistors. To prevent heating, we divide resistors into many short sections, each of which is heat-sunk small metal banks. rely on electron diffusion carry heat out and calculate...

We describe a technique for initializing the flux state of an inductively isolated Josephson junction, fulfilling essential requirement using device as qubit. By oscillating applied magnetic with specified amplitude and offset, we can select any allowed long-lived metastable states. this to $\mathrm{Nb}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{Nb}$ $\mathrm{Al}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{Al}$...

We analyze the effect of dissipation and low-frequency current noise on quantum coherence in a current-biased Josephson junction that has low damping. Developing stochastic Bloch equation for reduced density matrix system, we determine resonance response system to weak external microwave drive. characterize by finding spectroscopic time ${T}_{2}^{*}$ as function energy relaxation ${T}_{1}$ power spectral density. For with constant up frequency...

We report measurements of Rabi oscillations and spectroscopic coherence times in an $\text{Al}/{\text{AlO}}_{x}/\text{Al}$ three $\text{Nb}/{\text{AlO}}_{x}/\text{Nb}$ dc SQUID phase qubits. One junction the acts as a qubit other current-controlled nonlinear isolating inductor, allowing us to change coupling current bias leads situ by order magnitude. found that for Al qubit, time ${T}_{2}^{\ensuremath{\ast}}$ varied from 3 7 ns decay envelope had constant...

Quantum ordering, mixing, and mobility of isotopic hydrogen mixtures adsorbed on graphite (0001) is studied in the monolayer range by diffraction, small angle (SANS), quasielastic neutron scattering (QENS). Solid ${\mathrm{H}}_{2}\ensuremath{-}{\mathrm{D}}_{2}$ commensurate with substrate remain random from 18 K down to 3 K. Incommensurate solid at completion without a partial second layer top show tendency towards phase separation clustering as temperature reduced below 20 This monitored...

In an effort to determine the suitability of Josephson junctions for applications in quantum computation, we measured low-noise dc current-voltage characteristics Nb/AlO/sub x//Nb and Al/AlO/sub x//Al temperature range from 90 mK 1 K. Nb-based samples were obtained several different facilities with critical currents a few /spl mu/A current densities between 100 3000 A/cm/sup 2/. We fabricated Al-based using double-angle evaporation about 30 found that sub-gap leakage does not depend on K,...

When a Josephson junction in the zero-voltage state is current-biased below its critical current, rate that it escapes to finite-voltage depends on quantum of junction. By employing slow current sweep, possible observe experimentally emptying thermally populated first excited level as well-defined feature escape rate. This provides simple method determining junction's energy relaxation time ${T}_{1}$, key parameter for evaluating utility computation. We discuss temperature regime where this...

We describe how single Josephson junctions can be connected together capacitively to form a two-qubit system. discuss the general behavior of this system show energy level dependence on various junction parameters and choice coupling strengths. also measurement techniques for reading out both qubits which are relevant our ongoing experiments.

We calculate the effective capacitance of a Cooper-pair box and demonstrate that can be used as variable capacitor. The gate voltage modulates charge transfer through small junction in box, leading to gate-dependent capacitance. describe ongoing experiments use an Al/AlO/sub x//Al tunable circuit element at milli-Kelvin temperatures discuss possible applications quantum computing coupling between two Josephson qubits.

Several theoretical analyses of the two superconducting energy gaps magnesium diboride, ∆π and ∆σ, predict substructures within each gap, rather than pure numbers.Recent experiments have revealed similar structures.We report tunneling conductance data providing additional experimental evidence for these features.The absence features in c-axis tunneling, a sharp peak subgap (associated with counterelectrode material), support conclusion that are intrinsic to MgB2.By demonstrating inadequacy...

Graphene is a relatively new material (2004) made of atomic layers carbon arranged in honeycomb lattice. Josephson junction devices are from graphene by depositing two parallel superconducting leads on flake. These have hysteretic current-voltage characteristics with supercurrent branch and Shapiro steps appear when irradiated microwaves. properties motivate us to investigate the presence quantum metastable states similar those found conventional current-biased junctions. We present work...

In a university introductory physics laboratory, the measurement of time constants in RC and LR circuits usually employ conventional oscilloscope. The standard oscilloscope with its many knobs switches often intimidate first-time users can cause anxiety especially among non-physics non-engineering majors. On other hand, an iPad has more familiar, less intimidating friendlier touch-pad interface. We report our experience using as virtual algebra-based laboratory course. used commercial...

We report measurements of spectroscopic linewidth and Rabi oscillations in three thin-film dc SQUID phase qubits. One device had a 6-turn Nb loop, the second single-turn Al third was first order gradiometer formed from wound counter-wound coils to provide isolation spatially uniform flux noise. In 6-7.2 GHz range, coherence times for varied 4 ns 8 ns, about same as other devices (4 10 ns). The time constant decay significantly longer (20 30 ns) than either (10 15 These results imply that...

The energy states of a well-isolated hysteretic Josephson junction in the "phase" regime can be used as qubit. state determined by measuring when switches from zero-voltage (qubit) to running voltage state, since different levels have switching rates. experimental challenge is provide sufficient electrical isolation wires that must attached bias current and measure events. This effective at frequencies around level spacing, our case 3-8 GHz. We report on design measurement isolated...

Magnesium diboride has many intriguing characteristics, including its relatively high critical temperature and two-band nature. Most prior studies of MgB2 thin film Josephson junctions have been conducted above 2 Kelvin. We report results sub-1 Kelvin experiments MgB2/insulator/Pb whose a-b plane is exposed for electron tunneling. By measuring differential conductance at low temperature, new details in the structure sigma- pi-band gaps are observed this data, consistent with theoretical predictions.

We examine resonant activation from the zero-voltage state of a current-biased Josephson junction in limit low damping. In this limit, quantum dynamics can be described by master equation. Our finite temperature analysis includes transitions between any two levels potential well and accounts for escape near top well. results show that energy level quantization lightly damped junctions should observable even temperatures thermal regime, where only classical behavior is observed with moderate...

Microwave resonant activation is a powerful, straightforward technique to study classical and quantum systems, experimentally realized in Josephson junction devices cooled very low temperatures. These typically consist of two single-gap superconductors separated by weak link. We report the results first experiments on hybrid thin film junctions consisting multi-gap superconductor (MgB2) (Pb or Sn). can interpret plasma frequency terms theories both for conventional junctions. Using these...